The present invention relates to an optical transmitter module, and more particularly to an optical transmitter module using a flexible printed circuit board having flexibility to output a high frequency signal.
Optical transmitter modules include a direct modulation optical transmitter module and an electro-absorption modulation optical transmitter module. With the direct modulation, an electrical signal input from the outside is provided to a laser diode (hereinafter referred to as LD) and then an optical signal is output. With the EA modulation, an electrical signal is provided to an electro-absorption modulator (hereinafter referred to as EA modulator element) that modulates the laser beam output from the LD, and then an optical signal is output.
Recently the optical transmission modules have been miniaturized and integrated, due to the demand for reduction in power consumption and costs in addition to the increase in the data transmission speed. Further, the miniaturized optical transmitter module has been mounted on a transceiver, often by means of a flexible printed circuit board as a means for electrically connecting the inside and outside of the housing of the optical transmitter module in order to absorb the difference in the package configuration.
However, in the case of such an optical transmitter module using the flexible printed circuit board, the electrical signal propagates through a signal line of the flexible printed circuit board and thus the output optical signal is degraded. FIG. 1 shows frequency characteristics (S21) of the light output of the optical transmitter module to which the flexible printed circuit board is used. As shown in (a) of FIG. 1, there is a region in which the energy loss increases in the frequency response in a band of several GHz, and in which a dip appears with a depth of not less than 1 dB. At this time, in the output waveform (eye pattern), fluctuation (jitter) occurs in a time direction depending on the frequency response and the output amplitude is distorted. As a result, the eye opening is reduced.
The flexible printed circuit board includes a laser terminal pattern for supplying a drive current to the LD, a Peltier terminal pattern for supplying a current to a Peltier element, and the like, in addition to a signal pattern through which the transmitted electrical signal propagates. It is assumed that the dip appears due to the interaction between the signal pattern and the other patterns running alongside in the flexible printed circuit board.
Thus, a method of forming a shield can generally be used as an anti-dip measure. The method of forming a shield over the flexible printed circuit board is often used as an anti-noise measure in the module using the flexible printed circuit board. JP-A No. 2003-110207 describes a technology in which a shield is provided over the entire surface of the flexible printed circuit board. In fact, as shown in (b) of FIG. 1, the frequency characteristics are significantly improved when the flexible printed circuit board is shielded by conductors in the optical transmitter module.
In a method of shielding the transmission line by conductors, which is generally used as an anti-EMI measure, it is necessary for the signal pattern of the flexible printed circuit board to have a characteristic impedance of about 50Ω. For example, with a configuration in which the entire surfaces of the flexible printed circuit board are covered by conductive layers, generally the thickness of the flexible printed circuit board is not less than 500 μm due to the manufacturing limitation of the signal pattern (the minimum dimension: 50 μm). This leads not only to a loss of flexibility but also to a disadvantage in terms of manufacturing costs and manufacturing accuracy of the patterns.